Measuring apparatuses designed for continuously tracking a target point and coordinatively determining the position of said point can generally be combined under the term laser tracker. In this case, a target point can be represented by a retroreflective unit (e.g. cube prism) which is targeted by an optical measurement beam of the measuring apparatus, in particular a laser beam. The laser beam is reflected back to the measuring apparatus in a parallel fashion, the reflected beam being detected by a detection unit of the apparatus. In this case, firstly, an emission direction and respectively a reception direction of the beam are ascertained, for example by means of sensors for angle measurement which are assigned to a deflection mirror or a targeting unit of the system. In addition, with the detection of the beam, a distance from the measuring apparatus to the target point is ascertained, e.g. by means of propagation time or phase difference measurement, and—increasingly in a standardized manner in modern systems—an offset of the received beam from a zero position is ascertained on a sensor. By means of this offset that is measurable in this way, it is possible to determine a difference in position between the center of a retroreflector and the impingement point of the laser beam on the reflector and it is possible to correct or readjust the alignment of the laser beam depending on this deviation in such a way that the offset on the sensor is reduced, in particular is “zero” and the beam is thus aligned in the direction of the reflector center. As a result of the readjustment of the laser beam alignment, continuous target tracking of the target point can be carried out and the distance and position of the target point can be determined continuously relative to the measuring device. The readjustment can be realized in this case by means of a change in alignment of the deflection mirror provided for deflecting the laser beam, said deflection mirror being movable in a motorized manner, and/or by pivoting of the targeting unit having the beam-guiding laser optical unit.
Laser trackers according to the prior art can additionally be embodied with an optical image detection unit with a two-dimensional, light-sensitive array, e.g. a CCD or CID camera or a camera based on a CMOS array, or with a pixel array sensor and with an image processing unit. In this case, the laser tracker and the camera are mounted one on top of another in particular in such a way that their positions cannot be altered relative to one another. The camera is arranged, for example, in a manner rotatable together with the laser tracker about the substantially perpendicular axis thereof, but in a manner pivotable up and down independently of the laser tracker and thus, in particular, separately from the optical unit for the laser beam. In particular, the camera can have a fisheye optical unit and pivoting of the camera can thus be avoided, or the necessity thereof can at least be reduced, on account of a very large image detection range of the camera. Furthermore, the camera—e.g. depending on the respective application—can be embodied as pivotable only about one axis. In alternative embodiments, the camera can be installed in an integrated design together with the laser optical unit in a common housing.
With the detection and evaluation of an image—by means of an image detection and image processing unit—of a so-called auxiliary measuring instrument with markings whose relative position with respect to one another is known, it is thus possible to deduce an orientation of an object (e.g. a probe) arranged on the auxiliary measuring instrument in space. Together with the determined spatial position of the target point, it is furthermore possible to precisely determine the position and orientation of the object in space absolutely and/or relative to the laser tracker.
The object whose position and orientation are measured by means of the measuring device mentioned therefore need not be a measuring probe itself, for example, but rather can be the auxiliary measuring instrument. The latter, as part of the measuring system, for the measurement, is brought into a position that is mechanically defined relative to the target object or can be determined during the measurement, wherein, by means of the measured position and orientation of said instrument, it is possible to deduce the position and, if appropriate, the orientation of the measuring probe, for example.
Such auxiliary measuring instruments can be embodied by so-called contact sensing tools that are positioned with their contact point on a point of the target object. The contact sensing tool has markings, e.g. light points, and a reflector, which represents a target point on the contact sensing tool and can be targeted by the laser beam of the tracker, the positions of the markings and of the reflector relative to the contact point of the contact sensing tool being known precisely. The auxiliary measuring instrument can also be, in a manner known to a person skilled in the art, a, for example handheld, scanner equipped for distance measurement for contactless surface measurements, the direction and position of the scanner measurement beam used for the distance measurement relative to the light points and reflectors arranged on the scanner being known precisely. A scanner of this type is described in EP 0 553 266, for example.
For determining the orientation of the auxiliary measuring instrument, a detection direction of the camera is continuously aligned such that an image can be detected in the direction of the tracking beam of the laser tracker. The camera can furthermore have a zoom function, wherein a magnification level can be set depending on the determined distance between laser tracker and target point or auxiliary measuring instrument. With these two adaptation functions (alignment and magnification), the camera can thus continuously detect an image in which the auxiliary measuring instrument and, in particular, the light points of the auxiliary measuring instrument are imaged. An electronically evaluatable two-dimensional image of a spatial arrangement of light points arises as a result.
The image processing unit is provided for evaluating the image. This can be used to carry out identification of the imaged light points, determination of the centroids of the imaged light points and determination of the image coordinates of said centroids, from which, for example, solid angles between the optical axis of the sensor, in particular the detection direction, and the direction from the sensor to the respective light points can be calculated.
A measuring device of this type comprising a laser tracker and an image detection unit for determining the position and orientation of objects in space on which light points and reflectors are arranged is described in U.S. Pat. No. 5,973,788, for example.
With the use of such measuring devices, e.g. at least three light points which can be registered by the image detection unit and e.g. one reflector that reflects the measurement beam of the laser tracker can be arranged on the object whose position and orientation are to be determined, in known positions relative to the object. The light points to be registered by the image detection unit can be active light sources (e.g. light emitting diodes) or reflectors to be illuminated, wherein the light points can additionally be equipped or arranged in such a way that they are unambiguously distinguishable from one another. Alternatively, it is also known (depending on the required accuracy) to determine the position and orientation of such an auxiliary measuring instrument having light points only with the aid of camera images and the evaluation of the image positions of the recorded light points in the images, i.e. without additional assistance of a (propagation time or phase) distance measurement with a laser unit, as is e.g. also described in the publication documents EP 2 008 120 B1 and EP 2 010 941 B.
In general, a maximum achievable operating speed of these systems—besides the structural features of the tracker or camera—is essentially also determined by a process speed of the image analysis. The speed of performance of the image analysis and thus of the determination of the light point positions in the image is essentially limited by the image processing efficiency and by the volume of data that can be processed in a predefined time interval.
The image data processing speed thus constitutes a main bottleneck of laser trackers with a camera in the prior art. In this case, the image data are read out pixel by pixel and a brightness or color value is determined for each pixel. Each pixel which exceeds a predetermined threshold value is then taken into account in the calculation of respective image centroids with regard to the light points detected in the image. In conventional systems, the detection of the light points in the image is started after the read-out of all pixels of the image sensor (e.g. CMOS) or at least of all pixels which belong to an imaged light spot, and, for example, a respectively detected camera image is processed as a whole or a contiguous part of the image is processed as a whole. Moreover, in these systems as standard all pixels of the sensor are checked with regard to the respective exposure state. For each collection of pixels whose read-out signal lies above the threshold value and so they are illuminated by an identical light spot of the auxiliary measuring means, a centroid is then ascertained mathematically, wherein the position of the centroid in the image represents the position of the respective light point. By virtue of the fact that the determination of the centroids of the light points in the normal case begins only after the read-out of the entire area sensor and this evaluation requires a comparatively high degree of computational complexity and thus also a high degree of time expenditure, there are disturbing latencies between detecting an image and obtaining a result by evaluating the image, which also concerns a maximum achievable repetition rate for continuously recording images and, on the basis thereof, continuously determining the position of the auxiliary measuring instrument and readjusting the alignment of the camera. This can therefore greatly influence the determination and tracking of the orientation of a contact sensing tool (as auxiliary measuring instrument), in particular with regard to continuous position and orientation determination. As a consequence, as a result of this delay in the evaluation of the spatial light point arrangement, e.g. errors can occur in the determination of the exact position of a contact point of the contact sensing tool.
Accordingly, it is a general object of the present invention to provide a measuring apparatus and a measuring method for improved, faster and more reliable determination of a spatial orientation of an auxiliary measuring instrument.
It is a specific object of the present invention to provide, in the context of image detection with an area sensor, read-out of image information from the area sensor and determination of image positions of detected markings of the auxiliary measuring instrument, an improved and faster image evaluation method for images detected by means of the area sensor.